Illustrated herein, in various exemplary embodiments, are processes or methods for forming silane esters. In particular, the disclosure relates to a process for limiting the premature polymerization of a silane ester during its formation. The present disclosure is applicable with respect to the formation of materials suitable for use in the layers or components of an imaging member, photoconductor or photoreceptor that is used in forming xerographic or electrostatographic images. While the disclosure will is discussed with reference to such materials, the process or method is generally amenable to the formation of silane esters and to imaging members, etc., containing the same.
In an electrophotographic application such as xerography, a charge retentive surface (i.e., photoconductor, photoreceptor, or imaging surface) is electrostatically charged and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as “toner.” Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface.
The aforementioned process is known, and useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be image-wise discharged in a variety of ways. Ion projection devices where a charge is image-wise deposited on a charge retentive substrate operate similarly.
Electrophotographic imaging members are commonly multilayered photoreceptors that, in a negative charging system, include a substrate support, an optional electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer. The photoreceptor or imaging members can take several forms, including flexible belts, rigid drums, and the like.
Optional overcoating layers (i.e., overcoats) have been applied in order to protect the charge transport layers. The overcoating layer provides enhanced mechanical functions, such as for example enhancing the wear and scratch resistance, to enhance the service life of the imaging member.
Silane ester materials can be used as precursor materials for silicone-based hard coats or overcoats in imaging members or photoreceptors used in xerographic or electrostatographic printers. An example of a silane ester suitable for use in forming a silicone hard coat in a photoreceptor is the silane ester of Formula I:

The silane ester of Formula I is typically formed by reacting a salt of a dicarboxylic acid of Formula II:
with an alkyl halide containing a siloxane group. The dicarboxylic acid of Formula II is converted to the dipotassium salt by reaction with potassium carbonate in a DMF-toluene mixture. Water produced by the reaction is then removed azeotropically by toluene distiallation. Following the removal of water, the resulting dipotassium salt is reacted with a dialkyloxy silane that includes a halogenated alkyl group, such as, for example, 3-iodopropyl methyl diisopropoxy silane. The dipotassium salt undergoes a condensation reaction with the silane to form a silane ester, such as, for example, the silane ester of Formula I.
There are several limitations associated with the foregoing process. First, the removal of water is a crucial step in the process. The condensation reaction of the dipotassium salt of the carboxylic acid with the dialkoxy silane is very sensitive to water in that both one of the reactants and the resultant silane ester product are highly labile and will begin to prematurely polymerize in the presence of even a trace amount of water. Second, even with the removal of water from the dipotassium salt of the carboxylic acid, the silane ester resulting from the condensation reaction with the alkoxy silane always contains several impurities, including at least about 5 to about 10 percent of oligomer. Consequently, the resultant silane ester must be purified using column chromatography to remove the impurities. A drawback associated with this requirement is that column chromatographic methods, to treat these silane esters and remove the oligomer impurities, are not generally amenable or practical on industrial scales. In particular, column chromatography of silane esters result in increase cycle times and unit manufacturing costs. In particular, it may be costly to achieve or obtain photoreceptor grade products.
In view of the foregoing, it is desirable to provide a method or process for forming silane esters that reduces or eliminates certain impurities, such as, for example, oligomers, in the silane ester. Additionally, it is desirable to provide a process for forming silane esters that reduces the post reaction processing requirements to obtain an oligomer free silane ester product. Such products are useful in producing imaging members or photoreceptors.